The invention relates to applicators for applying microwaves to a material and for subjecting a material at least one of whose main components is a dielectric, particularly in the solid of liquid phase, to microwaves so as to impart to the material microwaves energy.
An applicator comprises essentially a waveguide whose inlet is coupled to a generator of microwaves. The waveguide also seres as a receptacle for the material being treated, or contains a receptacle. If the material is a solid, the body of material can itself have an inlet face through which penetrate the waves produced by the generator. On the other hand, if the material is a fluid, or for example is in the form of solid spherules, the inlet face is defined by the face of the receptacle which contains the material.
The yield of the apparatus, that is to say the ratio of the energy consumed in the waveguide to the energy supplied, is itself of great importance, but is also important because if too much energy is lost it is necessary to provide costly devices for absorbing the lost energy issuing from the outlet face of the material. To increase the yield it is possible, in principle, to use a very long waveguide. But this costly solution is sometimes quite impossible in practice. In particular, if the angle of loss of the dielectric, that is to say its tangent, is less than 0.05, the length of the reactor becomes inpracticable. It has been proposed to reflect a portion of the incident wave from the generator, after it has passed through the material in the waveguide and issues from the outlet face of the material as the outlet wave. It has also been proposed to reflect the wave leaving the inlet face of the material (this wave would otherwise return to the generator), reflecting the wave back into the applicator. For this purpose one can use a mirror or reflecting surface, that is to say an electrical conductor which forms a short circuit at hyper frequencies, the mirror being positioned at the outlet end of the waveguide or enclosure. A mirror of this kind usually takes the form of a metal plate or metal window installed at the outlet end of the waveguide or in an extension of the waveguide.
But this method for recovering energy which is not consumed in the applicator runs into certain difficulties.
The wave, which has passed once through the material and then been reflected from the other end, and has then passed through the material again but in the opposite direction, and has then been reflected again at the inlet end of the applicator by the coupling device, adds itself, in amplitude and phase, to the wave entering the applicator for the first time. The reflected wave has to reinforce the incident wave, rather than weakening it. This determines the length of the enclosure as a function of the wavelength emitted by the generator, the geometrical dimensions of the material being treated and its permitivity. The receptacle is therefore a unimode resonant cavity in the direction of the waveguide. The present invention utilizes this fact in homogenizing the treatment applied to the material.
A second difficulty, in recuperating the energy which is not consumed in the applicator, arises in that the coupling factor of the resonator, or enclosure, must be correctly chosen. The portion of the energy which penetrates into the applicator is exactly the same as the portion which issues from the applicator after each passage through it, the corresponding circuit being reciprocal. If the coupling aperture is too small, the small quantity of energy which penetrates into the resonator is reflected several times inside the resonator and not much leaves. But the yield of the applicator is low, because a great amount of energy is reflected by the inlet. On the other hand, if the coupling aperture is too large, the resonance is weekened by the energy lost through the aperture after each passage. The best coupling factor has to be found, so that the energy reflected by the aperture is exactly compensated by the energy which escapes from the resonator. Under these circumstances all the power delivered by the generator is consumed in the applicator. The matter can be expressed by saying that the length of the applicator is multiplied by a factor, called the resonance rise Q (which is also a measure of the ratio of the power stored to the power dissipated), and it can be said that a system of high stationary waves creates itself in the interior of the resonator.
But this system of stationary waves can be very inconvenient. In certain regions of the material, where the amplitude of the electromagnetic field reaches its peak, the material can be damaged. In other regions, a quarter of a wavelength away from the first regions, in the material or in the waveguide (1/4 g), the amplitude is minimal. In these regions the material is insufficiently treated. This heterogeneity of effect reduces the usefullness of the applicator.